Cartoon showing a simplified picture of hot, slowly expanding PDR region surrounding HD 37903. The surrounding cloud is shown in grey with denser gas having a darker shading. The hot molecular shell is shown in blue and the red shows the [C ii] PDR layer. In the southeast, where the C ii region expands into a dense, lumpy molecular cloud ridge the expansion is slowed down by the dense surrounding cloud, while it has expanded much further to the northwest. There are places where the PDR shell has broken through the cloud allowing radiation from the star to directly illuminate the cloud further out.